The present invention relates to ink jet printing apparatus and is concerned, more particularly, with preventing deterioration of print quality by ink jet printers by removal of air from printheads of the printing apparatus.
For sustained quality printing by drop-on-demand ink jet printers, it is well known that the printheads of the printers should be free of air pockets or bubbles, the printhead nozzle faces should be cleaned periodically, the printhead nozzle faces should be capped when the printer is not in use. If air bubbles or air pockets form inside the printheads, especially in the ink manifolds or reservoirs leading to the nozzles, they may reduce print quality, if sufficient in size, because the air pockets cause restricted ink flow. This restriction slows the refill of the passageways or channels to the nozzles to the point where droplet ejecting stability is severely compromised. Air is generally removed by priming the printhead at a maintenance station, such as for example, as disclosed in U.S. Pat. No. 5,404,158. However, in many instances, the air pockets remain even though ink is withdrawn from printhead. Dried ink residue and paper fibers, dust, and other foreign material can collect on the printhead nozzle face and affect droplet directionality, so that the printhead nozzle face should be cleaned periodically by, for example, a wiper blade as disclosed in U.S. Pat. Nos. 4,853,717 and 5,404,158. Capping the printhead nozzles is intended to prevent the ink exposed in the printhead nozzles from drying out. As disclosed in U.S. Pat. No. 5,339,102, the printhead nozzle face may be capped when the printer is not printing by, for example, moving the printhead from a printing zone to a maintenance station where movement of the carriage carrying the printhead causes the printhead nozzle face to be automatically capped. In U.S. Pat. No. 5,339,102, the attempt to remove air bubbles from the printhead is done by a priming operation, while the printhead nozzle face is capped at the maintenance station. Unfortunately, the withdrawal of ink does not always remove the air pockets from the printhead reservoir, with the result that some nozzles are starved of ink and fail to eject ink droplets.